1. Field of the Invention
The present invention relates to an improvement to a vehicular lamp lighting circuit that is employed in a vehicular lamp, such as a headlamp, a fog lamp, or a rear combination lamp.
2. Description of the Related Art
Conventionally, a vehicular lamp lighting circuit is known with a configuration that comprises a control unit 2 and a light emitting diode (LED) unit, i.e., a light source unit, 10, such as is depicted in FIG. 1, wherein the light emitting diode (LED) unit 10 is configured from an array of a plurality of an LED 9, and the control unit 2, which is broadly configured from a voltage boosting circuit 4, a reference voltage circuit 5, an amplifier 6, a voltage detection circuit 7, and a constant current circuit 8, performs a control of a turning on of electricity to the light emitting diode (LED) unit 10; refer, for instance, to Japanese Patent Laid Open No. 2003-187614.
In such a circumstance, if a voltage V of a battery should decline, for whatever reason, an output voltage V2 declines, a current IF, which flows through the LED 9, simultaneously is reduced, and a detected voltage V1 of an output current detected resistor R1 is reduced as well. As a consequence, a control voltage V3 is reduced, a difference arises between a reference voltage of the reference voltage circuit 5 and the control voltage V3, within a voltage boosting control IC, and, in order to reduce the difference within the voltage boosting control IC, a control acts in a direction that increases a duty ratio of a transistor of the voltage boosting circuit 4, the output voltage V2 is boosted and thereby is maintained at a value prior to the reduction thereof, and the current IF is preserved at a given level.
With regard to such a conventional circuit, however, if either of a plus terminal harness 10a or a minus terminal harness 10b, which connect the array of LED 9 to the control unit 2, should be grounded (GND) for whatever reason, a fault occurs as described below:
If the minus terminal harness 10b is caught by something and shorts out in a ground (GND), then, as is depicted in FIG. 2, the current IF flows, the detected voltage V1 that arises at both ends of the output current detected resistor R1 effectively falls to zero, causing the control voltage V3 to be reduced, whereupon the difference arises between the reference voltage of the reference voltage circuit 5 and the control voltage V3, within the voltage boosting control IC, and, in order to reduce the difference within the voltage boosting control IC, the control acts in the direction that increases the duty ratio of the transistor of the voltage boosting circuit 4, and the output voltage V2 is boosted, and repeating the control thereof over a plurality of instances gradually boosts the output voltage V2, increasing the current IF, such that the current IF that flows to the LED 9 significantly exceeds the constant current, and the light emitting diode (LED) unit 10 is destroyed as a result.
If the plus terminal harness 10a is caught by something and shorts out in the ground (GND), then, as is depicted in FIG. 3, the current IF would cease to flow to the light emitting diode (LED) unit 10, and thus, as with the shorting out of the minus terminal harness 10b, the detected voltage V1 effectively falls to zero, causing the control voltage V3 to be reduced, whereupon the difference arises between the reference voltage of the reference voltage circuit 5 and the control voltage V3, within the voltage boosting control IC, and, in order to reduce the difference within the voltage boosting control IC, the control acts in the direction that increases the duty ratio of the transistor of the voltage boosting circuit 4, and the output voltage V2 is boosted, and thus, the output voltage V2 is gradually boosted, increasing the current IF, and, while almost all of the current IF is going to ground (GND), thereby avoiding the destruction of the light emitting diode (LED) unit 10, repeating the control thereof over a plurality of instances causes the control unit 2 itself to be burnt out and thereby rendered unusable instead.